> while the internals of how the notifications get from e.g. the FCM backend to the notification drawer on your Android phone remain unspecified (and partly secret)
Actually, the web push standard strongly recommends that you use RFC 8030 for this, but it does allow browsers to substitute other protocols as long as their semantics are the same https://datatracker.ietf.org/doc/html/rfc8030. I believe Mozilla and Edge both use HTTP Push, I'm not sure what Google uses on Desktop Chrome but I could see it going either way. Safari probably uses APNS.
Actually, the web push standard strongly recommends that you use RFC 8030 for this, but it does allow browsers to substitute other protocols as long as their semantics are the same https://datatracker.ietf.org/doc/html/rfc8030. I believe Mozilla and Edge both use HTTP Push, I'm not sure what Google uses on Desktop Chrome but I could see it going either way. Safari probably uses APNS.